1. Field of the Invention
This invention relates to solid electrolyte fuel cells and more particularly to fuel cells having a solid acid electrolyte. Moreover, although not exclusively, the invention concerns an electrocatalyst support and electrode assembly for a solid electrolyte fuel cell.
2. Description of the Related Art
Currently there is much interest in fuel cells as a possible alternative clean energy source. As is known, a fuel cell combines hydrogen and oxygen to form water and produce a direct electrical current. A fuel cell comprises two electrodes, an anode and cathode, which are separated by an electrolyte. The electrolyte conducts ions (protons H+) between the cell electrodes but is impervious to electrons which flow between the electrodes around an external conduction path containing the load to complete the electrical circuit and to thereby provide electrical current flow to the load. In operation the fuel, molecular hydrogen, is oxidized by a catalyst at the anode (H2→2H++2e−) and molecular oxygen is reduced at the cathode to produce water (½O2+2H++2e−→H2O). These two half reactions are completed by the flow of ions (H+ protons) through the electrolyte and by the flow of electrons (e−) through the external circuit. Other fuels, such as methanol CH3OH4 or ethanol C2H5OH, can also be used to power the cell but have to be reformed to molecular hydrogen before providing them to the fuel cell.
There are a number of types of fuel cell and these are broadly categorized by the electrolyte membrane used in their construction. Common fuel cells include polymer electrolyte membrane fuel cells (PEMFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). More recently solid acid fuel cells (SAFCs) have been proposed in which the electrolyte comprises a superprotonic solid acid, such as CsH2PO4 (cesium dihydrogen phosphate CDP) which is a material which is partway between an acid and a salt. An example of a solid acid fuel cell membrane electrode assembly (MEA) 1 is shown in FIG. 1. As is known the MEA comprises a stack of members including: an anode electrode 2; a gas diffusion layer 3; an electrocatalyst layer 4, a solid acid electrolyte layer 5, a further electrocatalyst layer 6, a further gas diffusion layer 7 and a cathode electrode 8.
In the example illustrated the anode 2 and cathode 8 are made of stainless steel and have a hole 9 passing through their entire thickness to enable the introduction of fuel (hydrogen H2) and an oxidant (e.g. air, oxygen O2) into the fuel cell through the anode 2 and cathode 8 respectively and to allow fuel cell by-products (e.g. water H2O in the form of steam, CO2) to escape from the cell.
The gas diffusion layers 3, 7 typically comprise a porous ceramic material. The electrocatalyst 4, 6 which typically comprises a noble metal such as platinum or an alloy thereof is coated onto powdered carbon (carbon black) which functions as an electrocatalyst support. The solid acid electrolyte layer 5 comprises a solid salt. The cell 1 is constructed by physical stacking of the layers of the MEA, applying pressure to the assembly to ensure good contact between the electrocatalyst and electrolyte and enclosing the MEA within a gas tight enclosure (not shown).
US2006/0014068 and US2003/0104258 teach processes, techniques and compositions used to fabricate SAFC membrane electrode assemblies and US2006/0020070 discloses a SAFC electrolyte.
Potentially, SAFCs offer a number of advantages including a simplified construction since the electrolyte is in solid form and the ability to operate at intermediate temperatures in a range 150 to 350° C. The inventors have appreciated that in such cells the achievable power density is limited by the surface area of interfacing between the electrocatalyst and solid electrolyte. Moreover, due to the elevated operating temperature and by-products carbon corrosion of the carbon (carbon black) or graphite electrocatalyst support can reduce the life expectancy of the cell.